1. Field of the Invention
The present invention relates to a plastic optical fiber that can transmit light having wavelengths not only in the visible ray region, but also in the near infrared region, and that is heat-resistant. More specifically, the present invention relates to a plastic optical fiber that can be used as in optical fiber code and an optical fiber cable. 2. Description of the Related Art
Inorganic glass optical fibers have been known as optical fibers that are excellent in light transmission properties over a broad range of wavelengths. However, since they do not have good processability or flexural strength, plastic optical fibers were developed and have been widely used as optical fibers.
These plastic optical fibers are basically comprised of a core polymer made of a polymer that has excellent light transmission properties and a high refractive index, such as polymethylmethacrylate (hereinafter referred to as PMMA), polycarbonate (hereinafter referred to as PC), and a clad polymer made of a transparent polymer that has a refractive index below that of the core polymer, such as a fluorine-containing polymer.
Known examples of the plastic optical fibers of this type are optical fiber strands, bulk fibers made by covering optical fiber strands with a functional protective layer, optical fiber codes made by covering the optical fiber strands with a jacket, bundle fibers made of an assembly of the bulk fibers, and optical fiber cables made by applying tension members to the bulk fibers.
These plastic optical fibers, however, have many C--H bonds in the core polymer, and light absorption based on the expansion and contraction, or vibration of the C--H bonds, appears in the short wavelength regions. Five to eight times the harmonic absorption also appears in the near infrared to visible ray region, namely, at a wavelength of not less than 400 nm. The light transmission losses in these regions are serious. For example, the transmission loss of an optical fiber comprising a core polymer of PMMA is about 100 dB/Km at a wavelength of 650 nm, and about 400 dB/Km at a wavelength of 780 nm. To avoid the transmission losses based on the C--H bonds in the core polymer, a core polymer comprising d.sub.8 -PMMA, of which all the H atoms in the PMMA are replaced by D atoms, was proposed. This optical fiber containing d.sub.8 -PMMA as a core polymer has a transmission loss of 50 dB/Km at a wavelength of 780 nm. Deutrated PMMA, however, has high water-absorbing properties, and over time the d.sub.8 -PMMA core polymer absorbs water, and the transmission loss will increase over time. Thus, this optical fiber cannot be used as an optical fiber that is expected to have a high reliability over a long period.
Since LEDs that can emit rays in the near infrared region, and that have high power, and that can be used for high-speed data transmission, have been produced in large quantities at low cost, a plastic optical fiber that can effectively transmit rays in the near infrared region has been desired to be developed.